The best places for powder and tree skiing are Niseko and Hakkoda. Niseko is on Hokkaido, the northern island. Info here:
http://www.niseko.ne.jp/en/index.html
Hakkoda is in Aomori, on the northern tip of the main island, Honshu. It is near the capital, Aomori City. Site is only in Japanese, hopefully you know someone that can help you with it or can use a translator site:
http://www2u.biglobe.ne.jp/%7Eh-sansou/siki.html
For transportation to Niseko-Hirafu, take a flight to Chitose/Sapporo Airport, then a Bus/Train to Kutchan. For transportation to Aomori City, you can take the bullet train (shinkansen) from Tokyo after flying into Narita Intl. Airport. Or you can take a domestic flight from Haneda Airport in Tokyo. I prefer the bullet train, probably easier with gear too. If you do both and do Hakkoda first, you could also take a ferry from Aomori City across to Hakodate, Hokkaido, before the bus/train. Then after taking the bus/train back from Niseko, catch a domestic flight from Chitose/Sapporo Airport back to Haneda, switch over to Narita, and fly out. Just a suggestion.
I think I had my best skiing last year in February. Hope this helps.